The present invention relates to the art of cleaning sheet-like materials, and particularly fibrous materials, such as paper. More particularly, the present invention relates to a wet roller system for removing debris from paper in a high speed electrostatic printing device, such as a computer printer. A very thin film of viscous liquid is formed on two sequentially arranged rollers supporting the print receiving surface of the paper immediately prior to the print head, thereby removing debris from the paper just before the printing stage.
While the use of wetted objects are well-known for purposes of removing unwanted particles from surfaces, electrostatic printing, and particularly high speed electrostatic printing, has numerous special requirements that would normally dictate against the use of a wetted surface for removing or settling loose paper fibers or other debris on the paper immediately prior to printing. For example, the invention is designed specifically for use in a high speed computer printer utilizing liquid ink. Normally, if paper is wet when printed upon, the ink has a tendency to spread or feather upon contact with the paper resulting in an unacceptable low quality image. The present invention utilizes very thin liquid films to achieve substantial cleaning without appreciable deterioration of print quality.
Although paper dust, fibers and other debris on the surface of paper can lead to problems in any mechanical equipment handling or processing the paper, these problems are particularly acute in the case of electrostatic printers, and particularly in a high speed line printer designed to print-out computer data. In a high speed computer line printer constructed by the present assignee, a stream of air ions is modulated by passing the ions through small modulator apertures having an electrical field in them corresponding to data to be printed. The image modulated ion stream is attracted by an electrical field toward the print-out paper and passes through a mist of uncharged liquid ink droplets prior to striking the paper, whereupon the droplets become charged upon contact with the ions and are attracted toward the paper by the same field attracting the ions. The paper advances past the printing station at speeds up to and exceeding 10,000 lines of print per minute. Debris from the paper tends to clog the fine apertures of the ion modulating apparatus. Paper fibers that are deposited in the print area can also enter the ink cloud and act as collectors of the small ink droplets therein, eventually agglomerating sufficient ink to form an oversized droplet which could deposit on and objectionably mark the paper. The ink mist printing technique described above is disclosed in commonly assigned U.S. Pat. No. 3,779,166 of Pressman, et al., particularly at FIG. 8A thereof. The manner of forming an image modulated ion stream in a line printer is shown in commonly assigned U.S. Pat. No. 3,689,935 of Pressman, et al.
Several other techniques have been tried in an attempt to control paper particles and fibers in the subject computer printer. In one such technique, it was attempted to vacuum the paper before it entered the print receiving area; however, the vacuuming apparatus was bulky and noisy and, due to the small size of the debris particles, very high air flows were required in order to exert sufficient pressure on the particles to remove them from the paper. Additionally, it was discovered that if the paper surface is allowed to scrape against anything after leaving the vacuum area, additional particles and fibers are dislodged from the paper surface, thereby tending to defeat the vacuuming operation. Further, the vacuum tended to place an objectionably large degree of drag on the paper. While it is most desirable to place the cleaning station immediately prior to the printing station, this proved difficult or impossible to achieve with a vacuum cleaning technique because the vacuum air flows tended to distort the ink cloud at the printing station thereby disrupting printed image causing inferior quality printing.
Efforts to blow the dust particles from the paper with an air knife or the like were equally unsatisfactory inasmuch as this technique had essentially all the disadvantages of vacuuming, in addition to which it simply blew the particles into the atmosphere often to settle in some other undesirable location in the system.
Similarly, efforts to employ electrostatic cleaning techniques, such as by placing a positive charge on the paper and running it past a negative electrode prior to the printing stage, were equally unsatisfactory. Arcs were often produced by the presence of long fibers moving into the electrical field and these arcs tended to damage the thin corona wire. Further, difficulties were encountered in collecting the charged debris particles once they were removed from the paper.
Loose paper fibers and other debris from the paper surface has also been a problem in mechanical printers and other paper handling apparatus simply because the debris tends to clog any mechanism. Similar problems can exist in equipment for operating or handling any sheet-like material which is fibrous, or for any reason, which normally carries unwanted loose particulate matter on its surface.
Another problem encountered in electrostatic printing is that even though most loose particles are removed, dust or other small particles can be dislodged from the paper surface by an electrical field if the dust particles are themselves electrically charged.